This invention relates to a phase distribution measurement method and apparatus, phase correction method and apparatus, and magnetic resonance imaging method and apparatus, and more specifically, to a method and apparatus for measuring the phase distribution in an image obtained by magnetic resonance imaging, to a method and apparatus for correcting the phase of pixel data based on the measured phase distribution, and to a magnetic resonance imaging method and apparatus which obtains water and lipid images based on the phase-corrected pixel data.
In a magnetic resonance imaging apparatus, a stationary magnetic field is formed in a space which houses an imaging object, a gradient magnetic field and a high frequency magnetic field are formed in the stationary magnetic field space, and an image is formed (reconstituted) based on a magnetic resonance signal generated by the spin of the imaging object. Due to a chemical shift, the magnetic resonance signal of lipids has a different frequency from the magnetic resonance signal of water, so water and lipids can be imaged separately using a phase difference based on the frequency difference.
The phase of the magnetic resonance signal is affected by the non-uniformity of the stationary magnetic field intensity, so in order to image water and lipids separately without being affected by the magnetic field non-uniformity, a phase distribution, i.e. phase map, representing the stationary magnetic field non-uniformity, is calculated, and phase correction of the image is performed based thereon.
The phase map is obtained by calculating the phase of image data given by complex numbers for each pixel. To obtain a precise phase map, noise is first removed by processing the image with a lowpass filter.
FIGS. 1(A) and 1(B) show the concept of the phase map for a typical one-dimensional image. The origin of the phase map is taken as the center of the stationary magnetic field. The phase at the origin is 0. This diagram is a phase map when the stationary magnetic field intensity has a linear gradient, and the phase varies linearly according to the distance from the origin. When the phase exceeds +xcfx80, it returns to the xe2x88x92xcfx80 side, and when it exceeds xe2x88x92xcfx80, it returns to the +xcfx80 side, as shown by (a) in the figure. In other words, phase wrap around takes place.
Therefore, in parts where there is wraparound, a correction for wrap around, i.e. unwrapping, is performed to obtain a phase map without wraparound, as shown in FIG. 1(b). The presence or absence of wraparound is detected by whether or not the absolute value of the phase difference of the image data for adjacent pixels is 2xcfx80, and for pixels where wraparound is detected, 2xcfx80 is added to the phase with a reverse sign to that of the phase difference.
When lowpass filtering was performed on the image as a preliminary step to calculating the phase map, the pixel data was locally disturbed in some places resulting from change in the magnetization of the imaging object, phase variation due to lipids, ghosting due to blood circulation or body movement, or high noise. In these places, it appeared as if there were a wraparound phase state, but a precise phase map could not be obtained by performing unwrapping based thereon.
Further, if the magnetic field nonuniformity is nonlinear, the phase map comprises high order components. Lowpass filtering must be set so that these high order components are not removed, but this lowpass filtering setting is not easy to perform.
It is an object of this invention to provide a phase distribution measuring method and apparatus which can calculate a precise phase map even when there is local phase disturbance, to provide a phase correction method and device using this calculated phase map, and to produce a magnetic resonance imaging method and apparatus which implements this phase correction.
It is a further object of this invention to provide a phase correction method and apparatus which permits an easy phase correction including high frequency components, and to provide a magnetic resonance imaging method and apparatus which performs such a phase correction.
The invention from one aspect for resolving the aforesaid problems is a phase distribution measuring method, comprising the steps of lowpass filtering an image obtained by magnetic resonance imaging, detecting pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, calculating a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, and compensating by estimating the phase at said excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering, pixel positions where there is a phase disturbance are detected, a phase map is calculated excluding pixel data at these pixel positions, and compensation is then made by estimating the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies.
The invention from another aspect for resolving the above problems is a phase distribution measuring apparatus, comprising a filtering means which lowpass filters an image obtained by magnetic resonance imaging, a pixel position detecting means which detects pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, a phase distribution calculating means which calculates a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, and phase compensating means which compensates by estimating the phase at the excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering by the pixel position detecting means to detect pixel positions where there is a phase disturbance, a phase map is calculated excluding pixel data at these pixel positions by the phase distribution calculating means, and compensation is then made by the phase compensating means which estimates the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies.
The invention from another aspect for resolving the above problems is a phase correction method, comprising the steps of lowpass filtering an image obtained by magnetic resonance imaging, detecting pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, calculating a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, compensating by estimating the phase at the excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions, and performing phase correction of the image using the compensated phase distribution.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering, pixel positions where there is a phase disturbance are detected, a phase map is calculated excluding pixel data at these pixel positions, and compensation is made by estimating the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies. The image data is then phase-corrected using this phase map.
The invention from another aspect for resolving the above problems is a phase correction method, comprising the steps of lowpass filtering an image obtained by magnetic resonance imaging, calculating a phase distribution based on the lowpass filtered image, phase-correcting the image before lowpass filtering based on the calculated phase distribution, lowpass filtering the phase-corrected image again, calculating a new phase distribution based on the new lowpass filtered image, and performing a new phase correction of the phase-corrected image based on the new calculated phase distribution.
In the invention from this aspect, a first phase map is calculated from the lowpass filtered image, the image before lowpass filtering is phase-corrected based on this phase map, the phase-corrected image is lowpass filtered again, a second phase map is calculated from this image, and the image which was phase-corrected by the first phase map is then phase-corrected by the second phase map. By repeating this phase correction plural times, a phase map comprising high-order components is obtained.
The invention from another aspect for resolving the above problems is a phase correction apparatus, comprising a filtering means which lowpass filters an image obtained by magnetic resonance imaging, a pixel position detecting means which detects pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, a phase distribution calculating means which calculates a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, a phase compensating means which compensates by estimating the phase at the excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions, and a phase correcting means which performs a phase correction of the image using the compensated phase distribution.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering by the pixel position detecting means to detect pixel positions where there is a phase disturbance, a phase map is calculated excluding pixel data at these pixel positions by the phase distribution calculating means, and compensation is made by the phase compensating means which estimates the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies. The image data is then phase-corrected by the phase correcting means using this phase map.
The invention from another aspect for resolving the above problems is a phase correction apparatus, comprising a filtering means which lowpass filters an image obtained by magnetic resonance imaging, phase distribution calculating means which calculates a phase distribution based on the lowpass filtered image, phase correcting means which phase corrects the image before lowpass filtering based on the calculated phase distribution, and control means which lowpass filters the phase-corrected image again by the filtering means, calculates a new phase distribution by the phase distribution calculating means based on the new lowpass filtered image, and phase-corrects the phase-corrected image again by the phase correcting means based on the new calculated phase distribution.
In the invention from this aspect, a first phase map is calculated from the lowpass filtered image by the phase distribution calculating means, the image before lowpass filtering is phase-corrected based on this phase map by the phase correcting means, the phase-corrected image is lowpass filtered again, a second phase map is calculated from this image by the phase distribution calculating means, and the image which was phase-corrected by the first phase map is then phase-corrected by the second phase map, by the phase correcting means. By performing plural phase corrections in this way, a phase correction comprising high-order components is performed. The image data is then phase-corrected by the phase correcting means using this phase map.
The invention from another aspect for resolving the above problems is a magnetic resonance imaging apparatus, comprising an imaging means which acquires an image of an imaging object using magnetic resonance, a filtering means which lowpass filters the image obtained by magnetic resonance imaging, a pixel position detecting means which detects pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for the image before lowpass filtering and the image after lowpass filtering, a phase distribution calculating means which calculates a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, a phase compensating means which compensates by estimating the phase at the excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions, a phase correcting means which performs a phase correction of the image using the compensated phase distribution, and an image generating means which generates a water image and a lipid image separately using a phase difference of pixel data of the phase-corrected image.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering by the pixel position detecting means to detect pixel positions where there is a phase disturbance, a phase map is calculated excluding pixel data at these pixel positions by the phase distribution calculating means, and compensation is made by a phase compensating means which estimates the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies. The image data is phase-corrected by the phase correcting means using this phase map. Water and lipids are imaged separately by the image generating means based on the phase-corrected image data.
The invention from another aspect for resolving the above problems is a magnetic resonance imaging apparatus, comprising an imaging means which acquires an image of an imaging object using magnetic resonance, a filtering means which lowpass filters the image obtained by magnetic resonance imaging, a phase distribution calculating means which calculates a phase distribution based on the lowpass filtered image, a phase correcting means which phase-corrects the image before lowpass filtering based on the calculated phase distribution, a control means which lowpass filters the phase-corrected image again by the filtering means, calculates a new phase distribution by the phase distribution calculating means based on the new lowpass filtered image, and phase-corrects the phase-corrected image again by the phase correcting means based on the new calculated phase distribution, and an image generating means which generates a water image and a lipid image separately using a phase difference of pixel data of the phase-corrected image.
In the invention from this aspect, a first phase map is calculated from a lowpass filtered image by the phase distribution calculating means, the image before lowpass filtering is phase-corrected based on this phase map by the phase correcting means, the phase-corrected image is lowpass filtered again, a second phase map is calculated from this image by the phase distribution calculating means, and the image which was phase-corrected by the first phase map is then phase-corrected by the second phase map, by the phase correcting means. By performing phase correction plural times in this way, a phase correction comprising high-order components is performed. The image data is phase-corrected by the phase correcting means using this phase map. Water and lipids are imaged separately by the image generating means based on the phase-corrected image data.
The invention from another aspect for resolving the above problems is a magnetic resonance imaging method, comprising the steps of taking an image of an imaging object using magnetic resonance, lowpass filtering the image obtained by magnetic resonance imaging, detecting pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, calculating a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, compensating by estimating the phase at the excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions, performing phase correction of the image using the compensated phase distribution, and separately generating a water image and a lipid image using the phase difference of pixel data for the phase-corrected image.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering to detect pixel positions where there is a phase disturbance, a phase map is calculated excluding pixel data at these pixel positions, and compensation is made by estimating the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies. The image data is phase-corrected using this phase map. Water and lipids are imaged separately based on the phase-corrected image data.
The invention from another aspect for resolving the above problems is a magnetic resonance imaging method, comprising the steps of taking an image of an imaging object using magnetic resonance, lowpass filtering the image obtained by magnetic resonance imaging, calculating a phase distribution based on the lowpass filtered image, phase-correcting the image before lowpass filtering based on the calculated phase distribution, lowpass filtering the phase-corrected image again, calculating a new phase distribution based on the new lowpass filtered image, phase-correcting the phase-corrected image again based on the new calculated phase distribution, and generating a water image and a lipid image separately using a phase difference of pixel data of the phase-corrected image.
In the invention from this aspect, a first phase map is calculated from the lowpass filtered image, the image before lowpass filtering is phase-corrected based on this phase map, the phase-corrected image is lowpass filtered again, a second phase map is calculated from this image, and the image which was phase-corrected by the first phase map is then phase-corrected by the second phase map. By performing phase correction plural times in this way, a phase correction comprising high-order components is performed. The image data is phase-corrected by the phase correcting means using this phase map. Water and lipids are imaged separately by the image generating means based on the phase-corrected image data.
According to this invention, a phase distribution measuring method and apparatus which calculates a precise phase map even when there is a local phase disturbance, a phase correction method and apparatus using this calculated phase map, and a magnetic resonance imaging apparatus which performs this phase correction can be provided.
This invention also provides a phase correction method and apparatus which allow easy phase correction comprising also high frequency components, and a magnetic resonance imaging apparatus which performs such a phase correction, a phase distribution measuring method and apparatus which calculate a precise phase map with high performance, a phase correction method and apparatus using this calculated phase map, and a magnetic resonance imaging apparatus which performs this phase correction.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.